In the field of marine drives, there has for many years existed the problem of how to easily provide the capability for reverse thrust in ships for stopping, maneuvering and the like.
One option has been to provide a reverse gear which can be shifted to reverse rotation of the screw while the prime mover continues to rotate in its original direction. This is only a viable concept in small sizes, not in the thousands-of-horsepower range.
Another option is to stop and reverse the direction of the prime mover. Some marine diesels do this, as well as do steam turbines which have a few rows of reversing blading. This technique is not easily adaptable, however, to gas turbines.
A third option, and perhaps the option most frequently used in modern ships with gas turbine power, is to provide a variable pitch marine propeller, which will generate forward thrust, reverse thrust, or no thrust, depending upon the blade angle, all the while rotating in the same direction. This option also has a number of drawbacks. These variable pitch screws are complicated and expensive with pitch changing mechanism located in the screw hub and extending through a hollow shaft. The size of the screw hub, alone, results in a three to five percent efficiency penalty. The complexity of these screws results in a maintenance burden. Added to this is the fact that much of the mechanism is located outside the ship and the hollow hydrodynamic shaft is difficult to seal. Finally, there is a compromise in cruise efficiency because of the requirement to vary the blade pitch.
The present invention is directed to a simple means of effectively reversing the screw to generate reverse thrust without the attendant limitations of the existing options described above. The present invention utilizes a gas turbine prime mover provided with first and second low pressure, counter-rotating, power turbines. The first power turbine is operatively connected through appropriate gearing to the ship's shaft so as to provide forward screw rotation. The second power turbine is connected through appropriate gearing to the ship's shaft to provide reverse screw rotation. A brake assembly is provided for each power turbine so that only one of the power turbines effectively drives the ship's shaft at any given time.
The marine drive of the present invention requires no reverse gear. All of the gears in the drive of the present invention are in mesh at all times and are loaded. No reverse blading is necessary, nor the attendant difficulty of redirecting gas from the forward to the reverse blading. A conventional fixed pitch screw with a small hub may be employed without compromise, the screw being designed for optimum efficiency at cruise. All of the directional thrust determining mechanism is located within the ship and maintenance is greatly reduced and simplified. Finally, while it is difficult to seal the flow path gases with a rotating turbine case in an aircraft version of a dual rotation turbine, in a marine drive, 99% of the fuel is burned during forward operation when the turbine case is stationary. Zero-leakage seals may be pressed in place during this time. When reverse operation is desired, these seals are retracted and simple labyrinth seals contain the flow path gases. Leakage is not serious in reverse, as efficiency is not a significant consideration because of the short duty cycle.